An important contribution in this work was the clear delineation of packaging functionalities in the horticultural cold chain. Specifically, packaging systems must perform very different roles in three distinct components of the cold chain, namely, precooling (forced-air cooling; FAC), storage and refrigerated transport. Some examples include facilitating temperature control for quality preservation, removal of unwanted atmospheres (ethylene, CO2), intermittent-warming (to treat chilling
injury), phytosanitary treatments (either chemical or temperature related) and mechanical protection (to protect fruit from vibration, impact and compression forces).
Another major contribution in this thesis, was the elucidation and implementation of a multi-parameter evaluation approach for designing and assessing the performance of horticultural packaging, which is in contrast with previous work that based performance evaluations on just one or two parameters (e.g. cooling or airflow resistance). For example, a new performance parameter that combines the two main concerns facing FAC operators was introduced. Specifically, package performance was quantified with respect to cooling throughput (power usage versus cooling duration). Within this context, three new vent hole designs were proposed, using positioning schemes to ensure alignment during stacking (multi-scale). This positioning approach is applicable to many other packaging systems with different geometries, making the findings applicable to other packaging systems (e.g. citrus and stone fruit packages).
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The influence of trays versus loose fruit packing was also examined. This provides a vital link to the many previous studies investigating packaging packed with loose produce. Findings demonstrated that cartons packed with trays do not require larger vent hole areas and that significant improvements (48%) in cooling efficiency (energy consumption) are possible using alternative vent hole configurations. Next, the study examined the relationship between vent hole size, configuration and corrugated fibreboard material. This is an important component of carton design, since manufacturers often attempt to balance the ventilation size and fibreboard strength to achieve a desirable mechanical strength. Results showed that significant improvements in mechanical strength are possible, without decreasing cooling efficiency, using either different board types or different vent hole configurations in combination with smaller holes. The broad scope of this assessment further showed the effects of vent hole features (size, shape, position) are considerably more complex than previously assumed in literature. Increases in vent hole area generally resulted in a linear reduction in carton mechanical strength, which was consistent with previous work. However, the extent of improvement was dependent on which vent hole configuration was used. Curiously, the effect of both vent hole configuration and size on mechanical strength were also shown to have a significant interaction with the type of fibreboard used.
A key contribution in this study, was therefore: The observation that optimal vent hole design on cartons is affected by the type of fibreboard materials used, which has not been reported in literature before. This effect was shown to be related to the material properties of the corrugated fibreboard (e.g. thickness, rigidity and elasticity), which determined the mode of failure (buckling locations) of the carton. The implications of these findings are significant, since the material properties in corrugated fibreboard change throughout the lifetime of a carton, as moisture is adsorbed as a result of the high humidity conditions during cold storage (90-95% relative humidity; RH). Additionally, humidity cycles in the storage environment can also result in mechano-sorptive creep. However, most studies have only evaluated cartons using a single board type and at standard conditions (23 °C and 50% RH). A note of caution is that previous recommendations for a carton design based on these findings should, therefore, only be accepted within the examined range of materials and cold chain conditions investigated. Recommendations can therefore not be extrapolated to other cold chain conditions. Furthermore, these findings provide some explanation for the large discrepancies in carton performance, often reported between laboratory tests and field performance of new package designs. The results in this study clearly indicate that an improved mechanical testing approach, which incorporates the high moisture content distributions of cartons (during cold chain conditions) is critical for future evaluations. However, very little information is currently available in this area.
Another contribution in this thesis is the development and validation of a CFD model to predict moisture transport in corrugated fibreboard under RFC shipping conditions. Discussions with industry role players suggested that stacked cartons can fail even under optimal RFC conditions. The model was therefore used to characterise moisture contents and distributions in a pallet stack under these ideal transit conditions. Findings showed that the respective spatio-temporal moisture content gradients were relatively small. Furthermore, the main influence to moisture gradients was the presence of gradual long term temperature changes, whereas smaller factors (e.g. defrost cycles) were relatively insignificant. These conditions can be used as groundwork for future box compression test conditioning protocols, whereby the RFC conditions are replicated in a humidity chamber or in a well-controlled cold room. Additionally, the detailed simulation results can be used as boundary condition inputs for future numerical models evaluating carton mechanical strength.
A further contribution in this thesis, was the optimisation of RFC container space usage, which previously has not been considered in other studies. Current packaging systems (complimentary carton and pallet base design) do not use about 10% of the RFC floor area, representing a significant opportunity to improve cold chain efficiency. Two new improved packaging systems were proposed, where the optimal vent hole designs were guided based on findings from the preceding sections. The study extended the multi-parameter approach, by evaluating both horizontal (FAC) and vertical airflow (RFC) cooling. Although both designs showed promise, the “Tes” design, which stacks nine rectangular cartons per pallet (1 157 × 1 135 mm) was identified as a viable alternative to current package designs, showing improved cooling performance and 98.9% RFC floor area usage.